overhaul Box nodes

# Breaking Changes

- Message input properties are now expected to be within the `payload`,
  not hanging directly off of the message
- Credential format has changed and is now stored (as received) verbatim
  from the Box SDK
- `box watch` (events) Node no longer holds opinions about what the
  events look like--it no longer filters, modifies, or aggregates the
  events and the event data--the event data format now mirrors the Box
  API documentation (which means it's at least documented somewhere).
  Note that the Enterprise Event Stream events *have a different format*
  than the User Event Stream events!
- Essentially, existing flows and configurations will break, and this
  must be a major release.

# Enhancements

- Add OAuth2+JWT authentication mode for persistence
- Add "box folder items" Node to list the contents of a folder; accepts
  a folder ID, offset, and limit
- Add "box update file info" Node to modify file information (e.g.,
  tags)
- Add support for "download by file ID" in `box in` (download) Node
- Add support for file representations to `box in` Node; as appropriate,
  any download can be "downloaded as" plain text, PDF, and various-sized
  images
- Add support for Enterprise Event Streams in `box watch` (event) Node
- Add support for long-polling in User Event Streams in `box watch` Node
- Add an output to `box out` (upload) Node
- Add preflight checks to all uploads to avoid wasting time/bandwidth on
  large, failed uploads in `box out` Node
- Add automatic file creation and new version creation in `box out`
  Node; if a file does not exist it will be created, otherwise a new
  version will be created.
- Add stub for developer token authentication mode

# Refactors

- consume [Box's Node SDK](https://www.npmjs.com/package/box-node-sdk)
  directly
- move much logic into `lib/box-api.js`, which is the configuration Node
  itself; is now essentially an adapter for the Box Node SDK
- create `TokenStore` class, corresponding to interface defined by the
  Box Node SDK, to manage tokens for OAuth2 authentication--essentially
  an adapter to Node-RED's credentials system
- new palette and grid labels for various Nodes, since they were all
  previously the same
- split runtime `.js` files into one-per-Node; `box.js` now simply loads
  all files in `lib/`
- some ES2015+ enhancements
- use Promises where possible
- mixins provided for API functionality which changes depending on
  authentication mode
- some improvements to non-idiomatic JS
- some docstring improvements
- some locale string normalization

# Fixes / Other

- use canonical method to skip a suite in Box tests
- remove unused `should-sinon` from `package.json`
- update `package-lock.json`
- rewrite most tests due to changes in authentication and HTTP requests
  incurred by the use of the Box SDK; I would recommend actually testing
  at a higher level and stubbing SDK methods to avoid worrying about
  what the requests look like--we don't actually care, because that's
  the Box SDK's problem!
- add tests, though could use plenty more; the test helper isn't ready
  yet, and unfortunately I don't have time to work on it
- exempt `box/**/*.js` from JSHint checks, since I was fighting with it;
  see #225

Author: boneskull

.gitignore

@@ -1,5 +1,4 @@
 .npm
-.jshintignore
 .project
 .settings
 .tern-project

.jshintignore

@@ -0,0 +1 @@
+box/**/*.js